Rainfall

Rainfall in Ireland

Most of the eastern half of the country gets between 750 and 1000 (mm) of rainfall in the year. Rainfall in the west generally averages between 1000 and 1400 mm. In many mountainous districts rainfall exceeds 2000mm per year. The wettest months, in almost all areas are December and January. April is the driest month generally across the country. However, in many southern parts, June is the driest. Hail and snow contribute relatively little to the precipitation measured.

How Often Does it Rain?

The general impression is that it rains quite a lot of the time in Ireland, but two out of three hourly observations will not report any measurable rainfall. The average number of wet days (days 1mm or more of rain) ranges from about 150 days a year along the east and south east coasts, to about 225 days a year in parts of the west.

How Heavy is the Rain?

Unlike the rain in many other countries, especially in the tropics, average hourly rainfall amounts in Ireland are quite low, ranging from 1 to 2mm. Short-term rates can of course be much higher: for example, an hourly total of 10mm is not uncommon and totals of 15 to 20mm in an hour may be expected to occur once in 5 years. Hourly totals exceeding 25mm are rare in this country and when they do occur they are usually associated with heavy thunderstorms.

Information on the frequency of heavy rainfalls is often required by engineers, architects and others, usually in connection with design criteria for water management or drainage schemes. A depth duration frequency model allows for the estimation of point rainfall frequencies for a range of durations for any location in Ireland. For more information, see www.met.ie/climate/products03.asp .

Effects of Altitude

Annual amounts of precipitation increase by between 100 mm on eastern slopes and 200mm on exposed western slopes for every 100 m rise in altitude, the higher values occurring in the uplands of the west. At an altitude of 300 m, the annual rainfall may be as much as 50% greater than in the adjoining lowlands. Some summits in the southwest receive over 3000 mm annually. The increase in rainfall is accounted for by the general uplift of air over raised ground, which enhances both the intensity and duration of the rain and showers. On the other hand, many areas in the lee of mountains and hill ranges enjoy relatively lighter rainfalls because of protection from the prevailing westerly winds.

Measurements

Because rainfall is a key indicator of changes in the climate, measurements of rainfall are essential for assessing the affects of climate change on the water cycle and water balance. It has been measured at a number of locations in Ireland since the early 19th century. Following the establishment of the Irish Meteorological Service in 1936, the rainfall network was organised and expanded, reaching a peak of approximately 800 rainfall stations in the late 1950s.

Currently rainfall is recorded at synoptic and climate stations, in addition there is a wide network of voluntary rainfall observers. At synoptic stations, rainfall readings are made every hour on the hour; at climate and rainfall stations a daily rainfall total is recorded each day at 0900 UTC, the total recorded at this time is assigned to the previous day. There are also a number of rain gauges in remote locations which are read once a month, usually on the 1st. Met Éireann is in the process of automating its synoptic network of stations, the new generation of stations, known as TUCSON (The Unified Climate and Synoptic Observations Network), provides minute-by-minute readings of the main weather parameters, including rainfall. The map shows the locations of current rainfall stations, rainfall is also recorded at synoptic and climate stations.

Historically rainfall was measured by rain gauges which collect rainfall falling into a funnel of upper diameter 127mm, 305mm above the ground, these are still used in the voluntary rainfall Network. At Airports and TUCSON stations, rainfall is measured by 'tipping buckets', these tip when 0.1 or 0.2mm of rain falls, and the number of tips is counted. The rain recorder is surrounded by a turf wall (inset: traditional rain gauge).

Time Series and Trends: A number of locations have time-series longer than 100 years. The graph below shows the long term annual rainfall series for the stations at Valentia Observatory and at Dublin.

Data from rainfall stations are available in digital format from 1941, from these data gridded monthly rainfall totals have been produced for each month since January 1941. Below is the national annual rainfall time series, produced by averaging over all grid points for each year.

Seasonal Rainfall totals and difference from the 1961 to 1990 normals.

Rainfall shows great year to year variability. A 30 year running mean of the national annual rainfall indicated an increase in average national rainfall of approximately 70mm over the last two decades.

As with the annual totals of rainfall, all seasons show a small increase in totals over the last few decades.

Climate Change indicators

An expert team of the World Meteorological Organisation has described a range of rainfall and temperature climate change indicators, to enable uniform comparison of these variables. In the case of rainfall, these indices include the number of wet days and rain days and the number of days with rain above certain thresholds.

From individual station records, trends have been calculated for the indicators for stations which have a near-complete daily record for the 1961 to 2010 period. The trends for the number of days with daily rainfall greater than 10mm from 1961 to 2010 are shown above. The trends for rainfall do not show the same level of confidence as those for temperature, they show greater regional variation and occasionally conflicting trends from stations that are geographically relatively close.

Driving Rain

In calm conditions, raindrops fall vertically with a velocity that depends on their size. When a wind is blowing, the drops are also carried along horizontally and with strong winds deflections of more than 45 degrees from the vertical can occur, even for large drops. In such conditions, a vertical wall facing into the wind will receive more precipitation than a horizontal surface of equal area. During prolonged spells of wind-driven rain, brickwork and concrete may become saturated and rain may penetrate cracks in a structure or gaps around windows.

The amount of rain received by a wall correlates fairly well with the product of rainfall and the component of the wind speed normal to the wall. This led Lacy and Shellard to propose the product of the mean annual rainfall and the mean annual wind speed as a driving rain index which is proportional to the total rainfall driven in one year on to a vertical surface always facing the wind.